Metallic sandwich sheet

ABSTRACT

The invention concerns a sandwich-type metal sheet ( 2 ), adapted to a forming and welding operation, and exhibiting excellent resistance at high temperatures, comprising two metal sheet facings ( 1, 1 ′) having a melting point Tp, and a metallic core ( 4 ) having a melting point Ta, Ta capable of being equal to or different from Tp, whereby the core ( 4 ) has a density lower than the density of each of the facings ( 1, 1 ′), and the core ( 4 ) and each of the facings ( 1, 1 ′) are mutually bound by means of a metallic binding agent ( 3, 3 ′) having a melting point Tm lower than Ta and Tp. The invention also concerns a method for making said sandwich-type metal sheet ( 2 ), and its use in the automotive industry.

BACKGROUND OF THE INVENTION

The present invention relates to a sandwich sheet suitable for a formingand welding operation and exhibiting excellent resistance to hightemperatures, comprising two sheet metal facings and a metal core, toits manufacturing process and to its use for producing parts for motorvehicles.

In general, the main advantage of sandwich sheets over conventionalmetal sheets lies in the weight saving that can be achieved in theproduction of parts having predetermined mechanical strengthspecifications; this advantage is very important in automobileapplications, especially for reducing the fuel consumption of thevehicle.

Sandwich sheets used for the manufacture of parts for motor vehiclesmust meet not only welding and forming requirements, for example thoseperformed by drawing, bending and crimping, but also temperaturewithstand requirements. This is because during the operation of paintingthe sheet, which may include a prior anticorrosion treatment, the sheetis subjected to high temperatures and durations, generally between 140and 220° C. for 20 to 30 minutes, that are sufficient to allow bakingand/or crosslinking of the various coating layers applied.

DESCRIPTION OF THE PRIOR ART

For this purpose, sandwich sheets are known that comprise two sheetmetal facings joined together by a core formed by a polymer-based layerthat includes a continuous textile web of thermoplastic polymer fibers,impregnated with a thermoset polymer material for impregnating the weband for adhesion to the facings. These sheets exhibit good formabilityand good temperature withstand.

However, they have drawbacks, such as variations in the thickness of thesandwich sheet and defects in the adhesion of the textile web to thesheet metal facings, since the web is not uniform. However, above all,this type of sandwich sheet including a polymer are not weldable, or noteasily weldable, with in addition the release of toxic fumes, whichmakes them incompatible with use for the manufacture of parts for motorvehicles.

Apart from these drawbacks, the process for manufacturing such compositesandwich sheets does not allow acceptable productivity levels to beachieved, since the step of making the preimpregnated web adhere to thesheet metal facings is a very slow step.

Also known, from patent FR 2 767 088, are sandwich sheets composed oftwo steel facings linked together by a core made of a steel wool. Thesteel wool is joined to the steel facings by capacitor-discharge arcwelding. Admittedly such all-steel sandwich sheets are perfectlyweldable and withstand temperatures well above 160° C. However, theprocess for manufacturing such sheets cannot be carried out on anindustrial scale as, to weld the steel wool to the steel facing it isnecessary to weld the whole assembly at a temperature above 1300° C.Furthermore, at this temperature level the structure of the steelchanges, which is undesirable if it is intended to preserve themechanical properties and the formability of the steel facings and ofthe steel wool.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to remedy thesedrawbacks and to propose formable and weldable sandwich sheetsexhibiting good resistance to high temperatures and a good surfaceappearance, the manufacturing process for which is easy to implement.

For this purpose, the subject of the invention is a sandwich sheetsuitable for a forming and welding operation and exhibiting excellentresistance to high temperatures, comprising two sheet metal facingshaving a melting point T_(f); and a metal core having a melting pointT_(c), it being possible for T_(c) to be equal to or different fromT_(f), characterized in that the core has a density less than thedensity of each of the facings and in that the core and each of thefacings are bonded together by means of a metal bonding agent having amelting point T_(m) of less than T_(c) and less than T_(f).

The sandwich sheet according to the invention may also have thefollowing features:

the metal core occupies between 10 and 80% of the volume that separatesthe two sheet metal facings;

the metal core occupies between 20 and 60% of the volume that separatesthe two sheet metal facings;

the core consists of a metal wool, a knitted metal fabric, a woven metalfabric, a metal foam or a metal sponge;

the core is made of steel;

the sheet metal facings are made of steel; and

the sheet metal facings and the metal core are made of steel, and themetal bonding agent is chosen from tin and its alloys, zinc and itsalloys, and aluminum and its alloys.

The subject of the invention is also a process for manufacturing asandwich sheet, suitable for a forming and welding operation, andexhibiting excellent resistance to high temperatures, comprising twosheet metal facings having an internal face and an external face, andhaving a melting point T_(f), these being bonded together by a metalcore having a melting point T_(c), it being possible for T_(c) to beequal to or different from T_(f), the internal face of each of the twofacings being located so as to face the core, according to which itcomprises the steps consisting in:

inserting the metal core between the two sheet metal facings precoatedon their internal face with a metal coating, the melting point T_(coat)of which is below the melting point T_(f) of the sheet metal facing andbelow the melting point T_(c) of the metal core;

heating the assembly formed by the two sheet metal facings between whichthe metal core has been inserted at a temperature T lying between themelting point of the metal coating T_(coat) minus 50° C. and the meltingpoint of the metal coating T_(coat) plus 200° C., under speed andduration conditions such that the core adheres to each of the facings;and

cooling the assembly.

The process for manufacturing a sandwich sheet according to theinvention may also have the following features:

between the heating and cooling steps, pressure is applied to theassembly formed by the sheet metal facings and the metal core, saidpressure being adjusted so as not to damage the structure of the metalcore;

the assembly formed by the sheet metal facings and the metal core isheated by induction;

the thickness of the metal coating of each of the sheet metal facings isbetween 5 and 350 μm;

the thickness of the metal coating of each of the sheet metal facings isbetween 20 and 80 μm;

the rate at which the assembly formed by the sheet metal facings and themetal core is heated is greater than or equal to 30° C./s;

the time during which the assembly formed by the sheet metal facings andthe metal core is heated is less than 15 s;

the melting point of the coating T_(coat) is less than 0.9 times themelting point of the sheet metal facing T_(f) and less than 0.9 timesthe melting point of the metal core T_(c);

the internal face of each of the sheet metal facings is coated by hotdipping in a bath of liquid metal chosen from tin and its alloys, zincand its alloys and aluminum and its alloys;

the sheet metal facings are made of steel;

the density of the metal core is less than the density of each of thesheet metal facings;

the core consists of a metal wool, a knitted metal fabric, a woven metalfabric, a metal foam or a metal sponge; and

the external face of at least one of the two sheet metal facings (1, 1′)is coated with a coating, the melting point T_(e) of which is above themelting point of the metal coating that coats the internal face of eachof the two facings (1, 1′) T_(coat) plus 200° C.

The subject of the invention is also the use of the above sandwich sheetfor the production of automobile body parts that are formed, painted andthen heat treated.

The subject of the invention is also the heat treatment of these partsat above 160° C.

The features and advantages of the present invention will become moreclearly more apparent in the course of the following description, givenby way of non-limiting example, with reference to the single appendedFIGURE in which a sectional view of a sheet according to the inventionis shown.

DESCRIPTION OF THE DRAWING

The invention consists in using the metal layer coating the internalface of the sheet metal facing, the internal face of the facing beinglocated facing the core, as a brazing element in order to ensureadhesion between the sheet metal facing and the metal core.

As shown in the single FIGURE, each facing 1, 1′ of the sandwich sheet 2according to the invention consists of a metal sheet having a meltingpoint T_(f) and a thickness of between, for example, 0.1 mm and 2 mm,preferably between 0.2 mm and 0.5 mm. Preferably, steel sheets arechosen that have better mechanical properties than aluminum sheets andcan be drawn more easily. The type of steel will be chosen according tothe application to which the sandwich sheet 2 is intended. Thus, forexample, a carbon steel will be chosen to manufacture a hood or a door,while a stainless steel will be chosen to manufacture an exhaust linepipe.

The metal core 4 of the sandwich sheet 2 according to the invention hasa melting point T_(c) equal to or different from the melting point T_(f)of each of the facings 1, 1′, and a density less than that of thefacings 1, 1′ so as to lighten the sheet 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a preferred embodiment, the core 4 consists of a metal wool, aknitted metal fabric, a woven metal fabric, a metal foam or a metalsponge.

In another preferred embodiment, the core occupies between 10 and 80%,preferably between 20 and 60%, of the volume that separates the twosheet metal facings 1, 1′ and has a thickness of between 0.5 and 2 mm.

Above 80% occupation of the inter-facing volume, the weight saving ofthe resulting sandwich sheet 2 is insufficient.

Below 10% occupation of the inter-facing volume, the core 4 becomesfragile and risks being crushed during forming of the sandwich sheet 2.

Between 20 and 60% occupation of the inter-facing volume, the core 4offers a good compromise between weight saving and strength.

The material of the metal core 4 is preferably steel, either a carbonsteel or a stainless steel, as this material exhibits better formabilitythan, for example, aluminum.

The steel wool is obtained by shaving, preferably in the work-hardenedstate, in order to increase the mechanical properties of the structure.The strands of steel wool have a mean length of 70 cm and their diametermay vary from 50 to 500 μm. Preferably, the diameter of the steel woolstrands is about 200 μm. This diameter must be uniform in order to avoidany weakening, which would result in breakages.

The knitted steel fabric is formed from several steel threads with adiameter of between 10 and 50 μm, knitted together.

The woven steel fabric is also formed from one or more steel threadshaving a diameter of between 10 and 50 μm, woven together.

The adhesion between the core 4 and each of the facings 1, 1′ isprovided by a metal bonding agent 3, 3′ having a melting point T_(m)below the melting point of each of the facings T_(f) and below themelting point of the core T_(c).

When the core 4 and the two sheet metal facings 1, 1′ are made of steel,the metal bonding agent 3, 3′ is chosen from tin and its alloys, zincand its alloys, and aluminum and its alloys.

The metal bonding agent may be in the form either of a discontinuouslayer or, preferably, a continuous layer so as to ensure the bestpossible adhesion between the core and the two facings.

Thus, prior to the manufacture of the sandwich sheet, the internal faceof each of the two sheet metal facings is coated with a metal coatinghaving a melting point T_(coat) below the melting point of the sheetmetal facing T_(f) and below the melting point of the metal core T_(c),so that the coating, on melting, brazes the metal core. This is becauseit is essential, in order to assemble the sheet metal facings with themetal core by brazing, for the melting point of the metal coatingT_(coat), which acts here as a filler metal, to be below the meltingpoint of the facings T_(f) and below the melting point of the coreT_(c), so that the material of the coating melts well before thematerial of the core and the material of the sheet metal facings.

Preferably, so as to avoid any risk of the sheet metal facing 1, 1′ andthe metal core 4 melting in the event of excessive or overly longheating, the internal face of each of the two sheet metal facings 1, 1′is coated with a metal coating whose melting point T_(coat) ispreferably less than 0.9 times the melting point of the sheet metalT_(f) and less than 0.9 times the melting point of the metal core T_(c).

As an example, to form a sandwich sheet comprising two steel sheet metalfacings with a steel core, the facings are coated on at least one oftheir faces with a metal coating chosen from the following materials:

tin: melting point 230° C.;

zinc: melting point 420° C.;

zinc alloys, such as for example zinc-aluminum alloy containing about 5%aluminum by weight, the melting point of which is 381° C.; and

aluminum and its alloys, such as for example the aluminum-iron alloycontaining 2 to 4% iron by weight, the melting point of which is about660° C., or else aluminum-silicon alloys containing 5 to 11% iron byweight and 2 to 4% iron by weight, the melting point of which is about680° C.

A metal coating with a thickness of between 5 and 350 μm is deposited,on the internal face of each of the two sheet metal facings 1, 1′ thatis intended to face the core, either by electroplating or by vacuumdeposition or, preferably, by hot dipping in a liquid bath of thecoating material in the molten state.

If the thickness of the metal coating is less than 5 μm, there isinsufficient impregnation of the core 4 by the coating during brazingand consequently the adhesion between the sheet metal facing 1, 1′ andthe core 4 is insufficient, with the risk of the sandwich sheet 2delaminating.

On the other hand, coating the internal face of the facing with acoating of greater than 350 μm in thickness represents an additionalcost, and this also does not improve the adhesion between the core 4 andthe facings 1, 1′.

Preferably, the thickness of the metal coating is between 20 and 80 μmas, at this thickness level, the adhesion between the facings 1, 1′ andthe core 4 is sufficient for the sandwich sheet 2 not to delaminate,even after a severe forming operation, such as for example bending.

To manufacture the sandwich sheet 2, the procedure is as follows:

the metal core 4 is inserted between two sheet metal facings 1, 1′, theinternal face of which has been precoated, in such a way that the coatedinternal face of the facings 1, 1′ faces the core 4;

the whole assembly, formed by the two sheet metal facings 1, 1′ betweenwhich the metal core 4 has been inserted, is heated to a temperature Tof between the melting point of the metal coating T_(coat) minus 50° C.and the melting point of the metal coating T_(coat) plus 200° C.,preferably at a heating rate of not less than 30° C./s and preferablyfor a time of less than 15 s, so as to melt the material of the metalcoating; and

the assembly is cooled.

Upon softening or melting, the material of the metal coating brazes thefacing 1, 1′ onto the metal core 4 by impregnation of the surface of themetal core 4, thereby ensuring mechanical adhesion to the facings 1,1′/core 4 assembly. The softened or molten metallic material impregnatesthe metal strands making up the wool, the woven fabric or the knittedfabric of the core 4 that are located on the surface. If the assemblyformed by the two facings 1, 1′ and the core 4 is heated to atemperature T below the melting point of the metal coating T_(coat)minus 50° C., the material of the coating does not soften sufficientlyand does not impregnate the material of the core 4. At this temperaturelevel, brazing between the facing 1, 1′ and the core 4 is impossible andthe sandwich sheet 2 cannot be formed.

If the assembly is heated to a temperature T above the melting point ofthe metal coating T_(coat) plus 200° C., the material of the coatingthen runs the risk of boiling. Some of the coating material will escapefrom the sandwich sheet 2, forming drops on the free edges of thefacings. This makes the coating non-uniform and consequently preventsgood fastening of the facing 1, 1′ to the core 4.

Heating the assembly to a temperature T between the melting point of thecoating plus 50° C. and the melting point of the material plus 100° C.ensures good melting of the metal coating material and very goodadhesion of the core 4 to the facings 1, 1′ is obtained, without anyrisk of subsequent delamination and without any risk of the coatingmaterial escaping.

If the heating rate is less than 30° C./s, it is necessary to maintainthe heating temperature T_(coat) for a time of greater than 15 s inorder to completely soften or melt the coating material, such a timepenalizing the productivity.

When the heating time is greater than 15 s, the metal core composed ofwool, woven fabric, knitted fabric, foam or sponge starts to absorb someof the coating material, which causes the adhesion between the core 4and the facings 1, 1′ to deteriorate.

Preferably, the heating time is less than 3 s.

To heat the assembly formed by the facings 1, 1′ and the core 4, andconsequently to soften or melt the metal coating that coats the facings1, 1′, at least one heating element, such as for example an inductor, isplaced on either side of the assembly, and over its entire width.

According to another embodiment, the assembly formed by the facings 1,1′ and the core 4 is heated by calendering it between two heated rolls.

To improve the adhesion between the facings 1, 1′ and the core 4,pressure is applied to the sandwich sheet 2, the pressure being adjustedso as not to damage the structure of the core 4 and not to deform thefacings 1, 1′. The pressure applied to the sandwich sheet 2 must besufficiently low so that, for example, the metal wool or the knittedmetal fabric is not crushed and so that it thus maintains the integrityof its mechanical properties.

This pressure may be applied by the calendering rolls, by spraying afluid on either side of the facings, 1, 1′/core 4 assembly or else bymaking the facings 1, 1′/core 4 assembly run over a magnetized roll.

The embodiments of the sandwich sheet 2 according to the invention thathave just been described are not limiting.

Thus, prior to the manufacture of the sandwich sheet, it is alsopossible to coat the external face of at least one of the two sheetmetal facings 1, 1′ with a coating whose melting point T_(e) is abovethe melting point of the metal coating that coats the internal face ofeach of the two facings 1, 1′ T_(coat) plus 200° C. If it is desired toemploy facings 1, 1′ coated on their external face, it is necessary toensure that the coating used does not melt when the assembly formed bythe two facings between which the metal core 4 has been inserted isheated.

Typically, in this case the internal face of the facings 1, 1′ will becoated with a zinc layer and the external face of the facings 1, 1′ willbe coated with an aluminum-iron or aluminum-silicon layer.

However, usually the sheet metal facing 1, 1′ will be coated only on itsinternal face before the sandwich sheet 2 has been manufactured, and thecoating of the external face of the facings 1, 1′ will be carried out,for example in an electroplating bath, only when the sandwich sheet 2has been manufactured.

The forming of the sandwich sheet 2 according to the invention ispreferably carried out by drawing, bending or profiling. When the core 4consists of a metal exhibiting good ductility, such as for examplesteel, the sandwich sheet 2 according to the invention withstands severedrawing and/or bending and/or profiling conditions. This allows thesandwich sheet 2 according to the invention to be used to produce formedautomobile body parts such as, for example, hoods, doors or exhaust linepipes.

These parts are then, especially in the case of visible parts, paintedand then heat treated.

Prior to the painting operation, the sandwich sheet 2 may undergo ananticorrosion pretreatment of the cataphoretic type. The application ofthe paint can then be carried out by depositing powder (electrostatic,fluidized bed, etc.), or by the spraying or coating of a liquid paint insolution or of a molten polymer layer. Finally, the sandwich sheet 2 istreated at temperatures above 160° C., preferably above 180° C.,temperatures that are generally reached during the steps of curing ofthe anticorrosion pretreatments or of baking of the paint itself.

The sandwich sheets 2 obtained according to this process have, comparedwith sandwich sheets formed from a metal wool welded between two metalfacings, the following advantages:

a less energy-consumptive manufacturing process;

no modification of the structure of the sheet metal facings 1, 1′ and ofthe metal core 4. This is because, in a preferred embodiment of thesandwich sheet 2 according to the invention, in which the sheet metalfacings 1, 1′ are made of steel, the brazing of the steel facing to themetal core 4 is never carried out at a temperature above 700° C. Now, atthis temperature level the microstructure of the steel is not modifiedand it retains all its properties. For example, a dual-phase steel willpreserve its structure. According to the prior art, welding at atemperature of 1300° C. will melt the steel facing (and the metal core,if this is for example made of steel) and, during cooling, the steelruns the risk of losing its structure and therefore its mechanicalproperties. For example, a dual-phase steel heated to 1300° C. istransformed, when it cools, into martensite, which is hard and brittle,and therefore incapable of being drawn.

Furthermore, the sandwich sheets according to the invention have,compared with sandwich sheets consisting of a core comprising a polymer,the following advantages:

excellent weldability, with no discharge of volatile organic compounds;

excellent formability using various techniques, such as for example,bending, drawing or profiling, with no risk of delamination;

excellent resistance to high temperatures, especially temperatures above200° C., thereby allowing these sandwich sheets to be treated bycataphoresis; and

excellent recyclability.

Apart from temperature resistance and ease of forming, the sandwichsheets according to the invention have the following main advantages:

good mechanical properties in terms of stiffness of the sheets and ofthe parts produced by forming these sheets;

good mechanical properties of the formed parts, especially fatigueresistance and impact resistance; and

good surface appearance of the sheets obtained, even after forming.

1. A sandwich sheet (2) suitable for a forming and welding operation andexhibiting excellent resistance to high temperatures, comprising: twosheet metal facings (1, 1′) having a melting point T_(f); and a metalcore (4) having a melting point T_(c), it being possible for T_(c) to beequal to or different from T_(f), characterized in that the core (4) hasa density less than the density of each of the facings (1, 1′) and inthat the core (4) and each of the facings (1, 1′) are bonded together bymeans of a metal bonding agent (3, 3′) having a melting point T_(m) ofless than T_(c) and less than T_(f).
 2. The sandwich sheet (2) asclaimed in claim 1, characterized in that the metal core (4) occupiesbetween 10 and 80% of the volume that separates the two sheet metalfacings (1, 1′).
 3. The sandwich sheet (2) as claimed in claim 2,characterized in that the metal core (4) occupies between 20 and 60% ofthe volume that separates the two sheet metal facings (1, 1′).
 4. Thesandwich sheet (2) as claimed in any one of claims 1 to 3, characterizedin that the core (4) consists of a metal wool, a knitted metal fabric, awoven metal fabric, a metal foam or a metal sponge.
 5. The sandwichsheet (2) as claimed in any one of claims 1 to 4, characterized in thatthe core (4) is made of steel.
 6. The sandwich sheet (2) as claimed inany one of claims 1 to 5, characterized in that the sheet metal facings(1, 1′) are made of steel.
 7. The sandwich sheet (2) as claimed ineither of claims 5 and 6, characterized in that the sheet metal facings(1, 1′) and the metal core (4) are made of steel, and the metal bondingagent (3, 3′) is chosen from tin and its alloys, zinc and its alloys,and aluminum and its alloys.
 8. A process for manufacturing a sandwichsheet (2), suitable for a forming and welding operation, and exhibitingexcellent resistance to high temperatures, comprising two sheet metalfacings (1, 1′) having a melting point T_(f), these being bondedtogether by a metal core (4) having a melting point T_(c), it beingpossible for T_(c) to be equal to or different from T_(f), the core (4)having a density of less than the density of each of the facings (1,1′), characterized in that it comprises the steps consisting in:inserting the metal core (4) between the two sheet metal facings (1, 1′)precoated on at least one of their faces with a metal coating, themelting point T_(coat) of which is below the melting point T_(f) of thesheet metal facing and below the melting point T_(c) of the metal core,such that the coated face of each of the facings (1, 1′) faces the core(4); heating the assembly formed by the two sheet metal facings (1, 1′)between which the metal core (4) has been inserted at a temperature Tlying between the melting point of the metal coating T_(coat) minus 50°C. and the melting point of the metal coating T_(coat) plus 200° C.,under speed and duration conditions such that the core (4) adheres toeach of the facings (1, 1′); and cooling the assembly.
 9. The process asclaimed in claim 8, characterized in that, between the heating andcooling steps, pressure is applied to the assembly formed by the sheetmetal facings (1, 1′) and the metal core (4), said pressure beingadjusted so as not to damage the structure of the metal core (4). 10.The process as claimed in claim 8 or 9, characterized in that theassembly formed by the sheet metal facings (1, 1′) and the metal core(4) is heated by induction.
 11. The process as claimed in any one ofclaims 8 to 10, characterized in that the thickness of the metal coatingof each of the sheet metal facings (1, 1′) is between 5 and 350 μm. 12.The process as claimed in claim 11, characterized in that the thicknessof the metal coating of each of the sheet metal facings (1, 1′) isbetween 20 and 80 μm.
 13. The process as claimed in any one of claims 8to 12, characterized in that the rate at which the assembly formed bythe sheet metal facings (1, 1′) and the metal core (4) is heated isgreater than or equal to 30° C./s.
 14. The process as claimed in any oneof claims 8 to 13, characterized in that the time during which theassembly formed by the sheet metal facings (1, 1′) and the metal core(4) is heated is less than 15 s.
 15. The process as claimed in any oneof claims 8 to 14, characterized in that the melting point of thecoating T_(coat) is less than 0.9 times the melting point of the sheetmetal facing T_(f) and less than 0.9 times the melting point of themetal core T_(c).
 16. The process as claimed in any one of claims 8 to15, characterized in that the internal face of each of the sheet metalfacings (1, 1′) is coated by hot dipping in a bath of liquid metalchosen from tin and its alloys, zinc and its alloys and aluminum and itsalloys.
 17. The process as claimed in any one of claims 8 to 16,characterized in that the sheet metal facings (1, 1′) are made of steel.18. The process as claimed in any one of claims 8 to 17, characterizedin that the density of the metal core (4) is less than the density ofeach of the sheet metal facings (1, 1′).
 19. The process as claimed inclaim 18, characterized in that the core (4) consists of a metal wool, aknitted metal fabric, a woven metal fabric, a metal foam or a metalsponge.
 20. The process as claimed in any one of claims 8 to 19,characterized in that the external face of at least one of the two sheetmetal facings (1, 1′) is coated with a coating, the melting point T_(e)of which is above the melting point of the metal coating that coats theinternal face of each of the two facings (1, 1′) T_(coat) plus 200° C.21. A sandwich sheet that can be obtained by the process as claimed inany one of claims 8 to
 20. 22. The use of the sandwich sheet (2) asclaimed in any one of claims 1 to 7 and 21 for the production ofautomobile body parts formed, painted and then heat treated.
 23. A partobtained as claimed in claim 22, characterized in that it is heattreated at above 160° C.